The measurement of the contraction of the heart on an electrocardiogram (ECG) produces a waveform with characteristic elements which correspond to the various stages of contraction. One feature of an ECG is referred to as the QT interval, which represents the time period between the initiation of ventricular depolarization and completion of repolarization. The QT interval varies with the heart rate, age and gender. For example, the QT interval decreases with increasing heart rate. Men generally have shorter QT intervals than women.
Under certain circumstances, the QT interval can be prolonged, increasing the risk of a potentially fatal cardiac arrhythmia, resulting in the inability of the heart to contract effectively, which leads to a decrease in blood flow to periphery, including the brain, and syncope or sudden death. In rare cases, a prolonged QT interval is congenital and usually inherited. In other cases, prolongation of the QT interval is the result of a neurological disorder, such as stroke. Most frequently, a prolonged QT interval is caused by certain medications.
Congenital long QT syndrome (LQTS) comprises a distinct group of cardiac channelopathies characterized by QT prolongation on a 12-lead surface electrocardiogram (ECG) and increased risk for syncope, seizures, and sudden cardiac death (SCD) in the setting of a structurally normal heart and otherwise healthy individual. The incidence of LQTS may be as high as 1 in 2500 persons.
Because 2.5% of healthy individuals have a prolonged QT interval and 10-15% of LQTS patients have a normal QT interval, LQTS is not easily diagnosed and subsequently treated.